This section provides background information related to the present disclosure which is not necessarily prior art.
An engine's crankshaft converts reciprocating linear movement of a piston into rotational movement about a crank axis to provide torque to propel a vehicle, such as but not limited to a train, a boat, a plane, or an automobile. Crankshafts are a vital part of an engine, and are a starting point of engine design. Crankshaft design affects the overall packaging of the engine, and thereby the total mass of the engine. Accordingly, minimizing the size and/or mass of the crankshaft reduces the size and mass of the engine, which has a compounding effect on the overall size, mass and fuel economy of the vehicle.
The crankshaft includes at least one crank pin journal that is offset from the crank axis, to which a reciprocating piston is attached via a connecting rod. Force applied from the piston to the crankshaft through the offset connection therebetween generates torque in the crankshaft, which rotates the crankshaft about the crank axis. The crankshaft further includes at least one main bearing journal disposed concentrically about the crank axis. The crankshaft is secured to an engine block at the main bearing journals. A bearing is disposed about the main bearing journal, between the crankshaft and the engine block.
The crankshaft is typically formed or manufactured by a casting process, such as but not limited to a green sand casting process or a shell mold casting process, which uses cast iron to form the crankshaft. Alternatively, the crankshaft may be forged from a steel alloy. Steel is stronger than cast iron, and therefore is a more desirable material to use for crankshafts. However, the forging process is more costly than the casting process. Moreover, as indicated herein, it is often desirable to minimize the size and/or mass of the crankshaft to reduce the size and mass of the engine and the associated size and mass of the vehicle, and improve its fuel economy.
It is known that in order to reduce the weight of the crankshaft, the crank pins and main bearing journals may have hollow portions. The maximum volume of the hollow portions, and thus the weight reduction of the crankshaft, is limited by the stresses that must be borne by the crankshaft during engine operation. Stated differently, the crankshaft must have a sufficient thickness in order for the crankshaft to bear the stresses. In traditionally-casted crankshafts, such hollow portions are formed using one or more internal cores. However, when forging a crankshaft, to attain benefits only available via forging, it can be difficult to remove such hollow portions due to the need to machine the forged crankshaft.